This application relates to radio-frequency filters employed in communications systems, and more particularly to filters of the dual-mode, multiple-resonating-element type employed in communications systems operating at Ultra High Frequencies (UHF) and above.
Modern communications systems typically employ high-performance filters in antenna circuits and in other system components. Although communications systems which require such filters have been known for many years, a majority of such systems have heretofore been developed for use principally in military and aerospace applications, in which extremely high development and manufacturing costs have been acceptable in order to obtain high performance.
In recent years, however, several radio-based communications systems operating in the UHF frequency range and above have been developed for use primarily by commercial enterprises and individuals. A prime example of such systems is the cellular telephone network, which has enjoyed wide acceptance among commercial and individual subscribers. Such subscribers are generally more sensitive to costs than are military and aerospace users, and therefore, so are those who develop and operate such systems. Although the use of very-high-performance filters in lower-cost communications systems would be technically desirable, cost constraints have generally resulted in a tradeoff between filter performance and cost.
Broadly considered, "performance" encompasses a variety of preferred filter characteristics. In many cost-constrained systems, filter size is the aspect of performance which is sacrificed for cost. In the initial applications of modern communications systems, filter size may not have been critical, because usage levels were low and therefore did not require components to be densely implemented. However, as usage grew, the density of individual components had to increase to provide the desired coverage.
For example, in the early cellular telephone, systems, a relatively small number of fixed or base stations were provided, each having only a few channels, and such stations were generally installed in facilities at which space was not at a premium. However, as usage of cellular telephone systems has increased, the nature of base stations has changed. Individual base stations are now often provided with a larger number of channels than in the past. In addition, a substantially greater number of base stations may be provided. Because increased system density generally requires geographically smaller cells, the availability of suitable sites within each cell is limited, and many base stations must be located in facilities where space is at a premium. Thus, both of these changes in the nature of base stations put pressure on system providers to reduce the size of system equipment, including filters.
One type of filter preferred by some communications system builders is the dual-resonant-mode multiple-cavity filter. However, because the dimensions of a cavity determine its resonant frequencies, filters of this design tuned to a particular frequency are so large that they cannot be used in some applications.
Accordingly, communications systems designers have developed improved filters of reduced size for use in systems, such as communications satellites, in which size is an absolute constraint. S. J. Fiedziuszko ("Dual-Mode Dielectric Resonator Loaded Cavity Filters," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-309, No.9, September 1982, see pp. 1311-1316), Tang et al. (U.S. Pat. No. 4,652,843), and Chandra Kudsia ("Innovations in Microwave Filters and Multiplexing Networks for Communications Satellite Systems," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-40, No.6, June 1992, see pp. 1133, 1140-1141, and 1148), each disclose dual-mode multiple-cavity filters employing dielectric resonators within the cavities. In such cavities, the resonant frequencies are primarily determined by the dimensions and dielectric constant of the resonators in addition to the dimensions of the cavities. Compared to ordinary cavities, suitable resonator-loaded cavities of a particular resonant frequency can be dramatically smaller.
The design of cavity filters depends largely on the resonant frequencies of the resonant elements (cavities or resonators), and the couplings between them. In the above-referenced articles and patent, the couplings between resonant elements are controlled by conductive irises having cross-shaped slit apertures aligned with the orthogonal planes or axes associated with the selected resonant modes of the filter. In general, the amount of coupling between parallel modes of adjacent resonators or cavities is related to the size of the iris slit oriented in the direction corresponding to that mode. Irises constructed as taught in the prior art are predominantly aligned parallel to the magnetic field components of the resonances they couple together.
Although filters of this basic design have provided excellent performance, while requiring only a fraction of the volume of ordinary dual-mode cavity filters, they are expensive. A large contributor to the cost of these filters is the need to manufacture the irises with precision. In addition, because the irises are generally not easily adjustable once manufactured and installed in a filter, other elements of the filter must also be precisely manufactured, usually at great expense.
Zaki U.S. Pat. No. 5,083,102 discloses dual-mode dielectric-resonator filters which do not require irises. Zaki provides a cylindrical cavity filter having a plurality of coaxial, longitudinally-spaced, dielectric resonators. Coupling and adjustment screws extend into the cavity through the cavity wall and, depending on their relative locations, affect the resonant frequency of the resonators, the coupling between resonant modes in a single resonator, or the coupling between resonant modes in adjacent resonators.
Although Zaki's filters do not require irises, and therefore generally may be constructed less expensively than iris-based filter designs, Zaki's filters are not as space-efficient as equivalent iris-based filter designs. In general, the distance between resonators establishes a lower limit on the coupling between parallel resonant modes of adjacent resonators. Zaki's coupling screws generally increase the coupling. In many filter designs, it is necessary to provide very large differentials in the coupling between various modes. In Zaki's filters, in order to achieve large differentials in coupling between parallel resonant modes of adjacent resonators, it is necessary to space the resonators widely so that selected couplings could be acceptably low. This renders the resulting filter unacceptably large for many applications.